Metal wall framing systems have been used for some time. In such systems elongated metal elements --mullions, sills, jambs, etc.--grip the edges of opaque or transparent panels of various thicknesses to form a coherent wall, either of the store front type or of the curtain wall type. Typically each mullion, etc., consists of two primary parts, one on the interior side of the panels and one on the exterior. Various forms of auxiliary parts are employed to connect the primary parts together and to effect gripping of the panels.
In a completed wall system, one problem which has presented difficulty in the past is that of water intruded into the interior of the mullions. One source of intruded water is rainwater and window washing water which leaks past panel gripping gaskets. Another source is condensation from moist air within the mullions. Intruded water which is in the portions of the mullions that are exterior of the panels can be disposed of fairly readily by means of weep holes and the like. But intruded water which is in the portions of the mullions which are interior of the panels presents a serious disposal problem. it is aggravated in multi-story curtain wall structures, where a column of water a number of feet deep can readily accumulate in a vertical mullion. The interior panel gripping gasket confining such water will eventually leak under the pressure exerted by the column of water, and the water will be discharged into the interior of the building, which is always objectionable and often causes damage.
Another problem in completed wall systems is that of repair of broken panels (reglazing). In some wall systems, not only must the mullions immediately surrounding the broken panel be disassembled, but in addition, a significant portion of the undamaged surrounding wall area must be dismantled in order to effect a replacement of the broken panel.
In a curtain wall under construction, installers encounter difficulties with some wall systems in installing mullions in areas opposite ceilings or floor decks, where the cramped space available makes operations which are otherwise readily performed difficult and clumsy to execute.
In multi-story curtain walls, it is important to provide a wall systems in which the panels may be installed from within the building. If this is not done, installation expense is increased objectionably, since external scaffolding must be provided, erected, and dismantled as part of the wall installation procedure. Nonetheless, it is also important to be able to replace broken or damaged panels from outside the building, to avoid disturbing the occupants and interior furnishings.